1. Field of the Invention
In the field of precise instrumentation requiring auxiliary mechanical apparatus, the effects of vibration caused by operation of the auxiliary apparatus have been recognized as a serious problem for decades. The coupling of vibration from the apparatus frame to the instrument, or to a vehicle frame, has been the subject of much theoretical study and many experiments and developments. This work has been concentrated in two general areas: first, reduction of coupling of the vibratory movement of the apparatus to the instrument or the vehicle frame, through the use of vibration isolators or shock mounts; and secondly, cancellation of the effect of the vibration by causing an equal and opposite vibration to be applied either to the instrument or vehicle frame, or to be applied within the apparatus itself so that the apparatus frame has little or no vibration with respect to the instrument or the vehicle frame.
This invention relates particularly to the second type of device, which actually cancels the vibration or its effect.
A further problem which has been studied increasingly in recent decades has been the design of long life instrumentation, particularly for use where repair or other attention is virtually impossible, such as in unattended satellites. In these applications the results of wear of ordinary bearings, or even any rolling contact between metals, release microscopic particles which cause mechanical degradation of ultrasmooth surfaces or of small clearances, or other chemical-mechanical deterioration. To minimize these effects, contacts between elements which move with respect to each other, should be avoided wherever possible.
2. Description of the Prior Art
A first step in the reduction of vibration conducted to an instrument, vehicle frame, or other part of the environment has usually been to attempt to minimize the development of the initial vibration. Therefore every attempt is made to design mechanisms which are fully symmetrical, or are perfectly balanced with respect to rotational axes. Where the function of the machine requires the reciprocation of a mass such as a piston, the forces required to drive the piston couple an axial linear vibration into the machine frame. By the use of a linear drive motor coaxial with the piston, transverse vibrations can be eliminated.
If it is not convenient or possible to divide work between two pistons which are balanced and move in synchronization in opposite directions, the result is a linear vibration source that cannot be eliminated through improved manufacturing controls or improved design. This remaining vibration source must either be accepted at a level which is a function of the mass of the machine versus that of the vibrating piston, or cancelled for example by vibration absorption.
U.S. Pat. No. 4,483,425 discloses a system for attenuating or absorbing the vibration of a machine by vibration of a countermass which is also coupled to the machine frame. That system utilizes a feedback control system and an electrical linear drive motor to cause the piston to oscillate at a frequency and with a waveform which are predetermined or determined from some other sensing. In order to minimize reactive power flow to and from the linear motor, the preferred embodiment disclosed in this patent suggests the use of a spring which keeps the countermass centered about a fixed, approximately midstroke position. The spring-countermass combination can be tuned to the dominant vibration frequency, if the vibration to be cancelled has such a frequency.
When an attempt is made to construct such a machine like that in the '425 patent for use in a long life apparatus, the apparently simple problem of coupling the ends of the spring to the frame or the countermass turns out to be relatively complex. When a common helical spring is used to allow a fairly long linear stroke, movement of the last coil turn into and out of contact with the mount produces undesired non-linearities and also generates wear particles.
All the above expedients also suffer from the disadvantage that a mechanical spring and a mass have a single, fixed resonant frequency. Thus, good counterbalancing is only achieved if the vibration to be counteracted is at a given frequency or very close to that frequency. Small deviations can be handled by power supplied from an electrical or other motor coupled to the mass, but as soon as the frequency deviation becomes at all substantial, the power requirements become virtually as large as though there were no mechanical spring.